From: "Kenneth C. Jenks [GM2] (713" <kjenks@jsc.nasa.gov>
Subject: Elementary Ballistics
Newsgroups: sci.space,sci.physics
Henry Spencer (henry@zoo.toronto.edu) returned from the grave to write:
: (Actually, in practice, what you look up in a table is GM rather than
: G and M separately. The gravitational constant is a colossal pain to
: measure because gravity is so weak, and it is known to only about four
: digits. The GM of a planet will typically be known far more precisely
: than M, because GM can be determined directly from satellite orbits
: but getting M requires dividing GM by the poorly-known G.)
There have been a couple of proposals to go out to DEEP space and
measure G experimentally, but none has ever gotten beyond the "paper
study" stage. The general idea is to take a couple of BIG masses out
between the planets where space is relatively flat gravitaionally and
measure their mutual gravitational attraction, while measuring and
cancelling out all of the other forces at work on them (electromagetic,
electrostatic, solar wind, etc.). From the fundamental equation of
gravitational attraction, we could then determine G.
When you're done taking your measurements, you have some great big
masses of known chemical composition which you might steer into an
impact with an asteroid. We'd learn a lot about asteroid composition
by watching the resulting bang. Or you could drop them on the moon
just to watch them splash. That way we would KNOW how big a crater is
made by a meterorite of known mass and velocity. (This would help us
date certain areas of the moon better, since we use cratering densities
for many of our Lunar dating estimates.) Or you could nudge an
Earth-orbit-crossing asteroid a bit and make IT hit the moon.
(What uses can YOU think of for two big chunks of metal in space?)
Some day, we should get around to measuring G. It's one of the
fundamental constants of the Universe, and knowing its value to many
decimal places would aid astronomers somewhat. For example, we'd be
able to determine the masses of the planets. But LEO has too many
perturbing forces to make a near-Earth experiment worthwhile.
(Then again, maybe I'm wrong. There are some students out at CalTech
who think they can measure G in a Shuttle experiment. I wish them
well. If you're interested, you might drop a note to Ben McCall,
bjmccall@cco.caltech.edu.)
-- Ken Jenks, NASA/JSC/GM2, Space Shuttle Program Office
kjenks@gothamcity.jsc.nasa.gov (713) 483-4368
"Good ideas are common -- what's uncommon are people who'll
work hard enough to bring them about." -- Ashleigh Brilliant
------------------------------
Date: Tue, 26 Jan 1993 13:48:47 GMT
From: Ian Taylor <se_taylo@rcvie.co.at>
Subject: Galileo Laser Test Successful
Newsgroups: sci.space
In article <9301230715.AA00291@gap.cco.caltech.edu> News about Space from SEDS <SEDSNEWS%TAMVM1.BITNET@vm.usc.edu> writes:
> Operational use of this technology is anticipated some time
>after the year 2000, Lesh added.
> "We expect that the first deep space mission to fly optical
>will fly it as a mission enhancement experiment," said Lesh,
>"although this could change with the new emphasis on low-cost
>microspacecraft.
Interesting. What data rates are expected from this technology at
LEO/GEO/Lunar/Jovian/+ distances?
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